This invention relates in general to video disc players and in particular to apparatus for releasably securing a video disc for high speed rotation in such a player.
The record disc familiar to most is the conventional audio record, generally a 0.050 inch thick vinyl disc which is indexed by a spindle and supported by a rotatably driven turntable for presentation to a pick-up stylus. For all practical purposes there are but two types of audio discs on the market today, the 33 1/3 RPM long-play record and the 45 RPM short-play.
By way of contrast a video disc may be formed from 0.006 inch vinyl sheet, a construction which renders it extremely flexible and, to some extent, dictates the manner in which the disc is driven, or "flown," since it is partially supported for rotation upon a cushion of air.
Video discs also formed from vinyl sheet but having a thickness up to 0.044 inch are also "flown" on a similar manner. A video disc is further distinguished from an audio record by virtue of the fact that the former must store an enormous quantity of information in comparison to that impressed upon the audio record. To accommodate such information the center-to-center spacing between adjacent tracks on a video disc is in the order of 7 microns. By way of comparison, the center-to-center spacing between the tracks in an audio record is approximately 100 microns. For reasons pertinent to the signal processing techniques employed in retrieving the information stored on a video disc, the disc is driven at a speed substantially in excess of that at which audio discs are rotated, for example, a video disc of the type herein described is driven or flown at 1,800 RPM. However, it is recognized that it is also possible to fly video discs at 900 or even 450 RPM, speeds that still point up a significant difference between video and audio record play. It is therefore, obvious that the apparatus and techniques employed in playing audio records are not readily adaptable to play-back of video discs.
A playback deck specifically designed for video discs is described in U.S. Pat. No. 3,803,351, which was filed Aug. 1, 1972 in the names of Charles R. Pedersen and David S. Stewart, which patent is assigned to the same assigned as the present invention. As shown in that application, the video disc is indexed upon a hub and secured thereon by a removable magnetic type keeper, a component prone to misplacement. It is extremely important that the disc be firmly and releasably anchored to the hub since any variation in disc speed due to slippage between the disc and the hub will be manifested as incorrect information as the record track is monitored by the pick-up transducer. Accordingly, it is not only important that the disc be firmly secured to the hub but that provision be made for readily mounting and removing the disc. It is also important that the disc securing apparatus be arranged or so constructed as to preclude its loss or misplacement.
To resolve the misplacement problem, a built-in hub assembly was devised. Such a hub is described and claimed in U.S. Pat. No. 3,838,460 which issued to David S. Stewart on Sept. 24, 1974, which patent is also assigned to the assignee of the subject invention.